In various industrial machinery, there exists an important need to know accurately the mass flow in various processes, i.e. engine performance monitoring and optimization, process or production control, metering applications. A relevant application is the (petro)chemical industry, e.g. refineries, wherein large amounts of liquid flow between different process units that are dispersed over a large area and exposed to harsh environments often require constant monitoring from a centralized control room.
This is of specific interest to the maritime industry. In recent years, emissions requirements have become stricter. This trend will likely continue with governments looking to further reduced emissions from marine vessels in an effort to minimize the industry's environmental impact.
In order to further reduce emissions it is necessary to perform engine performance monitoring and optimization. Such techniques rely on accurate flow measurements in the fuel supply process.
The Coriolis effect is a widely known effect that can be used, e.g., for highly accurate measurement of flow parameters to be extracted accurately, such as mass flow rate, density, and volume flow rate. Its advantages include the ability to measure mass flow rate directly, as opposed to volume flow rate, and therefore being independent of the fluctuations in density, i.e. due to temperature variations or chemical composition changes. The devices to measure different parameters of a fluid flowing through a pipe by using the Coriolis Effect will be described herein as Coriolis flow meters.
Basically, Coriolis flow meters function on the principle of oscillating a (pair of) pipe(s) and measuring the oscillation amplitude, frequency and phase at different locations of the pipe and correlating that to the mass flow rate and fluid density.
In practice, Coriolis flow meters are implemented using a single pipe oscillating relative to reference frame, using double oscillating pipes with parallel flows with preferably 180° phase difference between the oscillations induced in the pipes, using double oscillating pipes with anti-parallel flow, in phase oscillations, using various alternative geometric arrangements of pipes such as a straight, a curved, a U-shaped, a V-shaped, a triangle-shaped, an Omega-shaped, a S-shaped or a Z-shaped pipe, or using rotating pipes.
Generally, a Coriolis force results in the effect of a mass moving in an established direction and then being forced to change direction with a vector component normal to the established direction of flow. In a vibrating system the direction change is constantly varying. Hence the Coriolis force is also constantly changing. A dynamic twisting motion occurs in addition to the oscillating motion, caused by the vibrating action. By measuring this twisting motion, a mass flow measurement can be obtained.
Typically, this measurement is made by sensors, which are placed at locations on the tube where the displacement variation in the tube due to the Coriolis force is the greatest. Two data values are derived from the sensor measurement. First a phase lag between one location of the tube and another location of the tube is calculated whereby the first and second locations of measurement are selected by having a difference in angular velocity with respect to the flow directions in the said two locations, such that a difference in Coriolis force at the said locations is different. This is indicative of the relative mass flow. Additionally, the resonant frequency of tube relates to the relative density of the measured material. Generally these measurements are temperature compensated.
Coriolis flow meters utilizing electrical detection systems suffer from some of the known limitations of electrical sensors. Namely, effects from electrical signal cross-talk, electromagnetic fields, limitations in operation conditions such as temperature, need for local and high accuracy signal processing for ensuring high meter accuracy. Especially the latter need for high accuracy localized signal processing for each unit results in high cost for each measurement point.
Furthermore, conditions in a fuel supply system may be hazardous. The flow meters and sensing electronics may be exposed to high temperatures and hazardous/corrosive fluids. In certain application, the sensing electronics may form an explosion hazard.
It is noted that patent publications U.S. Pat. Nos. 7,117,751 and 6,722,209 disclose a Coriolis flow meter wherein an optical read-out is realized using free-space detection techniques such as a Fabry-Perot interferometer or by detecting the tube in a light path or with quadrant detection. In practice, free space optics solutions suffer from a need for high precision high stability alignment of various optical components. These components are prone to contamination in an environment that either interferes with the guided light or results in contamination forming on the reflective or transmitting surfaces resulting in optical signal loss and drift over time.
Also, Coriolis flow meters are known wherein a fiber is attached in bent shape between an oscillating pair of tubes such that the distance change between the two pipes results in a change of the bend radius of the fiber which is then detectable through the change in optical transmission. However, optical transmission loss measurements are very inaccurate. Additionally, in order to analyze optical transmission loss, it is necessary to obtain well-correlated information regarding the optical source output power level and the power losses along the transmission fibers.